How do I Paint on has an Image that is used as Texture?
To paint on a mesh in Texture Paint Mode you need a UV unwrapped mesh.

Then, there are a couple of options:


Paint on a texture using the UV editor.

Enable Paint Mode and press T to bring up tools for painting

The image below shows where you toggle paint mode on



Choose the tool you want to use, and paint directly on the image.




(Please see this link for more info)


Paint using the 3D viewport.

Enable paint mode, select your tool and paint directly over the mesh. 




IMPORTANT: After you've painted on the texture you need to save the image on the UV/Image Editor. Use Save Image on the menu or AltS to overwrite the original image with the changes or use F3 to save as a different file)



If you don't save the image and close blender, all of your painted texures will disappear forever

You might also find this pages from the blender manual useful:

Texture Painting

UV Unwrapping a Mesh